WO2013000866 discloses an interferometric distance sensing device with a common path architecture. As is well known, an interferometric distance sensing device detects interference between a reference beam and a sensing beam that has been reflected from a target. In common path architecture, the reference beam travels through a large part of the path of the sensing beam towards the target and back. Typically, the device contains a reference mirror close to the sensor head that reflects the reference beam, but passes the sensing beam back and forth to the target. Thus, the sensing beam and the reference beam share a common path up to the reference mirror and back, and the part of the path of the sensing beam between the mirror and the target is the only difference between the reference and sensing paths. Compared to a conventional interferometer with separate sensing and reference paths, this eliminates effects of differences in temperature or mechanical variation of the common paths.
WO2013000866 modifies the common path architecture by the introduction of two parallel beam paths in the common path, so that both the reference beam and the sensing beam at the mirror are composed of a sum of parts that have traveled through the respective parallel beam paths (as used herein, “parallel” refers to the fact that respective parts of the light travels through both paths (parallelism of function) and not to parallel in a geometric sense). One of the parallel beam paths is longer than the other. This introduces the dependence on coherence length of the light source. Interference can only occur and be detected if the difference between the length of the light path of the sensing beam and the reference beam is not much more than the coherence length. In the conventional common path architecture this limits the distance from the mirror to the target to about the coherence length. With the parallel beam paths, this distance increased by half the difference between the parallel beam paths.
For the parallel beam path, as a result of the detected interference is due to a part of the reference beam that has traveled through the longer of the parallel beam paths and a part of the sensing beam that has traveled through the shorter of the parallel beam paths. Hence, part of the advantage of the common path architecture is lost, but the advantages still apply for the fiber to the sensing head, which is most exposed to environmental disturbances. WO2013000866 considers various positions for the location of the parallel beam paths: it can be located in the path between the light source and the mirror. When the path between the light source and the mirror contains an circulator that directs returning light to a detector, the parallel beam paths can be located in the path between the light source and the mirror before or after the circulator, or in the path from the circulator to the detector. In the latter case, the light to the target does not travel through the parallel paths.
Outside the field of common path architectures, sub-wavelength accurate path length difference measurement is possible when different paths can be coupled to different inputs of a three way coupler, as described for example in US2005275846.
As noted, common path architecture eliminates effects of differences in temperature or mechanical variation of the paths. Nevertheless, variation of the common path may still affect sensing results in conventional interferometer. Basically, the interference intensity as a function of distance to the target is the sum of a constant term and a periodic term, the phase position in this period depending on a ratio between the distance and the wavelength. By counting the number of periods, wavelength accuracy is possible. The distance could be determined with sub-wavelength accuracy from the intensity obtained the common path architecture and hence the phase measurement, when the amplitudes of the constant term and the periodic term would be known. However, variation of the common path affects these amplitudes, and hence the determination of the distance. This effect can be addressed by sensing at different wavelengths, but this complicates sensing.
WO2006/080923 discloses a simultaneous phase shifting Fizeau interferometer. It discusses the problems of encoding reference and test beams, so that they can be spatially separated at the back end. WO2006/080923 utilizes a tilted relationship between a reference and test mirror of a Fizeau interferometer to spatially separate the reflections. The separated beams are filtered to provide different polarization states and recombined to form a collinear beam. Alternatively, the beams may be injected at different angles into the Fizeau cavity. WO2006/080923 uses path length differences to suppress effects of spurious reflections.
In one embodiment, a delay line and a polarizing beam splitter are used in combination with a phase shifting interferometer to characterize a test surface in the Fizeau interferometer configuration. In this embodiment two mirrors are used in parallel to form a beam that is a combination of components of different delays. This combined beam is fed into an input of the interferometer.